This post is one of ten appearing in the series 10 Ways To Improve Your Landscape Photography.
For years I found the common advice of "focus 1/3 into the frame" a little puzzling. Understanding the reasoning behind that can really help you improve your focusing technique.

When you focus the camera, you are moving the camera's focal plane. If you're familiar with depth of field you'll be aware that more than just objects at the focal point will be sharp enough to be considered "in focus". Interestingly the acceptable focus will reach 1/3 in front of the focus point and 2/3 behind it.

Knowing this should make the "focus 1/3 in" advice make sense. If we focused on the first object we want to be sharp in our image, then 1/3 of the depth of field is in front of that object, on objects we don't care about. By focusing 1/3 beyond that object it becomes the first object in focus and we also maximise the usage of the remaining 2/3 of depth of field for the midground and background.

Conventional wisdom suggests shooting landscapes at f/16 or f/22 but these apertures can result in less sharp images overall due to lens construction and the effects of something called "diffraction". Most lenses are at their sharpest around f/8-f/11 so if we can use good technique to ensure everything from foreground to background is in focus at these apertures so much the better.

You can use a depth of field calculator (http://www.dofmaster.com is the most popular) to work out what depth of field a given aperture and focal distance will give you to maximise your foreground to background sharpness.

In practical terms I will often focus on the nearest object, note the measurement on the lens' distance scale and then manually refocus at about 2x the measurement (if e.g. if I'm focused at 1m, then I'll refocus at are just before 2m). I'll then use a combination of experience, calculator, DoF preview, LiveView and trial and error to get the focus just right.